Building pressure control is an increasingly important aspect of modern HVAC systems. Building pressure reflects the ratio of outside air entering the building envelope to the exhaust air leaving the envelope. Further, modern building designs often require variable air volume (VAV) systems that supply a constant temperature air (e.g., 55 degrees F.) to the building's supply ductwork. The temperature and air flow in individual areas or zones within the building are controlled by positioning dampers or boxes that allow more or less air into the space depending on the load.
Proper operation of a supply VAV system relies on a constant (i.e., static) pressure being maintained in the supply duct. Such a constant pressure ensures a sufficient supply of conditioned air on demand. Thus, VAV systems use static pressure control to sense and maintain supply duct pressure. VAV systems may achieve proper return air control by employing volumetric flow synchronization (volumetric flow control). Volumetric control systems modulate the return fan based off of actual supply and return fan air flow readings.
In the prior art, volumetric fan control was both complex and very costly, resulting in building owners and engineers opting for less costly, less complex, and consequently less effective systems. However, as the concern for indoor air quality has increased in recent years, so too has the concern for effective HVAC control. One area receiving a great deal of attention is the so-called "sick building syndrome." Further, new guidelines for indoor air quality have forced engineers and building owners to install systems that allow them to accurately control the mix of outside and inside air in compliance with those guidelines. ASHARE-62 IAQ is an example one such guideline.
Prior art volumetric fan control systems do not adequately meet these new building demands in a cost effective manner. First, prior art volumetric fan control systems require separate control panels, drives and probes. Second, these prior art systems require substantial field installation and set-up. Thus, prior art systems result in, inter alia, increased installation and operating costs. Further, due to the nature of integrating these separate systems, it may be difficult or impracticable for the specifying engineer and/or building owner to show compliance with ASHARE-62 IAQ guidelines.
As indicated above, most HVAC systems employ one or more fans in the supply and return ducts. In the interest of efficiency, as well as pressure and volume control, any given fan is not continuously operated. When a fan is idle (not powered), it may, however, continue to spin either from momentum or from persistent air flow within the duct. This unpowered spinning is referred to as "windmilling." Further, windmilling can occur in both a forward direction (e.g., the normally desired direction of rotation), or a reverse direction (e.g., the normally undesired direction of rotation).
Prior art HVAC systems do not account for this windmilling when powering or depowering fan motors. Applying power to a fan that is spinning in the reverse direction can cause excess torque forces that stress the fan and/or fan motor. Starting a fan motor when it is spinning in the reverse direction may also cause an overload and trip the applied adjustable speed drive.
There is a need, therefore, for an improved, cost effective, and energy efficient HVAC fan control system. In particular, there is a need for an HVAC fan control system the provides a unitary control and drive panel with reduced installation costs.